The present invention relates to power line communication (PLC) systems and particularly to a PLC system for communicating over existing residential wiring.
Numerous attempts to communicate over electrical power distribution conductors have been made in the past. The most successful of these have been those communication systems utilizing conductors of power distribution networks which can be readily tailored to accommodate successful communication, such as electric railway and primary power distribution networks. However, attempts to communicate over the power lines within a building have not generally been successful. The most significant problem encountered is noise. Since prior art approaches to communicating over residential branch circuits have traditionally utilized the line and neutral conductors as the communication links, noise generated by connected loads poses a significant obstacle to successful communication. The most common source of noise in residential power circuits are current switching devices which can be either of the mechanical or solid-state variety. Typically such noise is basically of a transient nature, but may be generated repetitively, as in the case of series motors and lamp dimmers. Noise can appear as voltages between line and neutral conductors, as well as currents flowing therethrough. Voltage magnitudes are typically in the range of 100 to 300 volts with frequency components ranging from 60 Hertz to hundreds of megaHertz. The internal reactance of the utilities' distribution transformer feeding the residential power circuits results in a transient disturbance of the line voltage with each switch opening and closing having frequency components within a typical PLC signal frequency band. In the case of a solid-state lamp dimmer, this disturbance may exceed 100 volts on each half-cycle of the 60 Hertz voltage wave. The large amplitude, fast rise-time (relative to PLC signal frequency), and repetitive nature of lamp dimmer noise are particularly troublesome to successful communication.
Furthermore, mechanical switches, such as timers, thermostats and motor brushes, transiently become sparkgap relaxation oscillators typically producing 300 volt disturbances at repetition rates within the PLC signal band and having frequency components extending to several hundred megaHertz. Such noise may be isolated bursts lasting a few milliseconds to repetitive trains, as in the case of series motors. The large amplitudes and wide range of frequencies of these noise disturbances pose a real challenge to receiver design.
Compounding the noise problem, PLC signal strength can vary widely throughout the building, especially between the two legs of the 120/240 volt electrical service, and is heavily dependent on the loads connected into the branch circuits. In general, inductive loads such as motors present a relatively high impedance to PLC signals. Resistive loads may vary widely from, for example, 144 ohms for a 100 Watt incandescent lamp to 12 ohms for a 1200 Watt toaster. The presence of a significant shunt capacitance in residential loads has not been prevalent in the past but is likely to increase. A load having a 0.1 microfarad capacitor imposes a 10 ohm shunt to PLC signals at a frequency of 160 kHz.
Increasing the transmitter's signal power to improve signal to noise ratio and provide successful signal reception throughout a building exacerbates the problem of PLC signals escaping the building via the line and neutral cables of the service entry drop. When these PLC signals reach the distribution transformer, they are blocked by the high internal transformer impedance. However, they can readily propagate along commonly connected service drops into other buildings fed thereby. It will be appreciated that what is signal to one PLC system is noise to all other PLC systems and the worst form of noise at that. Thus, PLC signals escaping one PLC system via a service drop and entering another PLC system via a commonly connected service drop poses especially serious noise problems for the latter system. Of course, signal traps may be utilized in the service entry drops to block such signal escape; however, to be effective, such traps are necessarily large and expensive.
Another problem in using the branch circuit line and neutral conductors to signal over is the necessity for some form of coupling impedance to enable communication between the two legs of the 120/240 volt electrical service installed in most buildings. Such coupling impedance is primarily provided by wiring capacitances and any 240 volt loads, such as ranges, water heaters, clothes dryers, etc. The internal impedance of the distribution transformer can rarely provide the requisite signal coupling impedance between the service legs. However, if a PLC system avails itself of this distribution transformer coupling impedance, its signals are available to invade other buildings as noise to any PLC system installed therein.
In the context of PLC systems for communicating between an electrical utility and its power-consuming customers for such purposes as remote meter reading, it has been proposed to utilize the distribution network neutral conductor and ground return as the communication link. This approach, exemplified in U.S. Pat. Nos. 3,702,460 and 4,016,429, is seen to avoid the impedance matching and signal attenuating problems posed by the continuously varying network loads connected between the phase or line conductors and neutral. Since a communication link utilizing the neutral conductor of a utilities' distribution network involves a multiplicity grounds, the neutral conductor being connected to ground at the service entry load center of each customer, the PLC systems described in these patents cannot be adapted to accommodate communication over branch circuits within a building inasmuch as the system neutral is necessarily grounded at only one point.
PLC systems have also been proposed for communicating over the branch circuits within a large building, such as a hotel, wherein PLC signals are coupled onto the neutral conductors with respect to ground, as evidenced by U.S. Pat. Nos. 2,743,434 and 3,810,096. The systems disclosed therein rely on the imposition of an inductive reactance of negligible impedance at the 60 Hertz power frequency between neutral and ground so as to provide, at the elevated PLC signal frequency, sufficient impedance separating the branch circuit neutral conductor terminations at the service entrance from ground potential to support an adequate signal voltage on the branch circuit neutral conductors with respect to ground. However, when this is done, the PLC signals can readily propagate out onto the neutral cable of the service drop and thence to the distribution transformer where they are coupled as noise onto other commonly connected service drop neutral cables. Since the PLC systems of these patents contemplate large buildings, this is probably not a problem as, in all likelihood, there are no other service drops commonly connected at the distribution transformer.
It is accordingly an object of the present invention to provide an improved power line communication system for signalling over residential branch circuits.
An additional object is to provide a power line communication system of the above character wherein signal integrity is relatively unaffected by branch circuit loads and any noise generated thereby.
A further object is to provide a power line communication system of the above character wherein signal strength can be made essentially uniform throughout the system of branch residential circuits.
Yet another object is to provide a power line communication system wherein the escape of signals out onto the service entry drop is effectively avoided.
Another object is to provide a power line communication system of the above character which is economically and conveniently adaptable to existing residential power circuits.
Other objects of the invention will in part be obvious and in part appear hereinafter.